The hydrologic, or water, cycle is the continuous, interlinked circulation of water among its various compartments in the environment. Hydrologic budgets are analyses of the quantities of water stored, and the rates of transfer into and out of those various compartments. A simplified hydrologic cycle starts with heating caused by solar energy and progresses through stages of evaporation (or sublimation), condensation, precipitation (snow, rain, hail, glaze), groundwater, and runoff.

The most important places in which water occurs are the oceans, glaciers, underground aquifers, surface waters, and the atmosphere. The total amount of water among all of these compartments is a fixed, global quantity. However, water moves readily among its various compartments through the processes of evaporation, precipitation, and surface and subsurface flows. Each of these compartments receives inputs of water and has corresponding outputs, representing a flow-through system. If there are imbalances between inputs and outputs...